Conventional hydrothermal process versus microwave-assisted hydrothermal synthesis of La1−xAgxMnO3+δ (x = 0, 0.2) perovskites used in methane combustion

A study on the synthesis of La_1−xAg_xMnO_3+δ (x = 0, 0.2) using a microwave process (MWhyd) has been carried out by comparing the heating time and reaction temperature with the same factor under conventional thermal process (CHhyd). Experiments have been conducted using the hydrothermal method at medium pressure (T = 200 °C, P = 20 atm) followed by a thermal treatment of the precursor at 700 °C (10 h).Structural and physico-chemical properties of the catalysts were investigated using X-ray diffraction (XRD), BET sorption, temperature-programmed reduction or desorption, mass spectrometry (TPR-MS and TPD-MS), and X-ray photoelectron spectroscopy (XPS). While CHhyd and MWhyd powder catalysts exhibited the same XRD patterns indexed as pure perovskite structure, their surface physico-chemical properties were found to be strongly influenced by the preparation method. The effect of the nature of oxygen species, their amount and mobility, evidenced by temperature programmed experiments, on the catalytic properties in methane combustion in the presence and in the absence of hydrogen sulphide has been studied. MWhyd-La_0.8Ag_0.2MnO_3+δ catalysts were found to exhibit a much better performance in methane combustion together with higher resistance to sulphur poisoning than CHhyd catalysts.     

Phase equilibria and crystal structure of the solid solution LaFe1−xNixO3−δ (0 ≤ x ≤ 1)

Phase equilibria in the LaFeO₃–“LaNiO₃” were studied at 1100 °C in air. The samples were synthesized by standard ceramic and/or solution route via nitrate or citrate precursors. According to the results of XRD it was found that the homogeneity ranges of LaFe_1−xNi_xO_3−δ solid solution lay within 0.0 ≤ x ≤ 0.4 (sp.gr. Pbnm) and 0.6 ≤ x ≤ 0.8 (sp.gr. ). The structural parameters (bond lengths, atom coordinates) for the single-phase samples were refined using Rietveld analysis. The unit cell parameters versus LaFe_1−xNi_xO_3−δ composition are presented.     

Critical behavior and magnetocaloric effect in La0.67Ca0.33Mn1−xCrxO3 (x = 0.1, 0.25)

Universal behavior of the magnetocaloric effect along with structural and critical exponent analysis in mixed manganite La_0.67Ca_0.33Mn_1−xCr_xO₃ (x = 0.1, 0.25), [LCMCr_0.1 and LCMCr_0.25] exhibiting second order phase transition are investigated. Structural study using Reitveld refinement of XRD patterns indicates orthorhombic structure with Pnma space group. Modified Arrott plot method has been adopted to study the critical behavior of the compounds at their transition region, which gives values of β = 0.555(6), γ = 1.17(4) and δ = 2.7096(7) at T_C = 232.5 K for LCMCr_0.1 and β = 0.68 (1), γ = 1.09(3) and δ = 2.9362(4) at T_C = 202.5 K for LCMCr_0.25. The values are close to those expected for mean field ferromagnets with long range order. With increase in Cr content, the temperature corresponding to the maximum entropy change as well as the magnetic transition temperature gradually shifts to low temperatures. The maximum magnetic entropy change was found to be 3.5 J/kg K for x = 0.1 and 2.2 J/kg K for x = 0.25 for a field change of 5 T. The field dependence of the magnetic entropy change is also analyzed, which shows the power law dependence namely, ?S_M ∝ Hⁿ, n = 0.9086(5) at T_C = 232.5 K and n = 0.849(7) at T_C = 202.5 K for LCMCr_0.1 and LCMCr_0.25 respectively. Relative cooling power was found to be about 147 J/kg for LCMCr_0.1 and 88 J/kg for LCMCr_0.25. The field dependence of the relative cooling power for both the compounds shows a H^1+1/δ dependence with the δ values in agreement with the mean field model.     

Oxygen production using La0.6Sr0.4Co0.2Fe0.8O3−α (LSCF) perovskite hollow fibre membrane modules

A phase-inversion/sintering technique has been employed in the production of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−α (LSCF) hollow fibre membranes, a bundle of which has then been placed in a high-temperature furnace for production of high purity oxygen from air at temperatures between 980 °C and 1060 °C. By applying a vacuum in the hollow fibre lumens, a product stream containing oxygen purity of 97.15% has been obtained. The downstream vacuum degree higher than 99 kPa shows negligible effect on the oxygen production rate. Studies on long-term operation suggest that the LSCF hollow fibre membranes are less stable for the oxygen production due to the segregation of the constituent membrane elements and the formation of new phases on the outer membrane surfaces. The effect of the operating cycle on the retrogression of membrane performance is much larger than that of duration used in a single cycle.     

Synthesis of ternary and quaternary CuInxGa1−xSe2 (0 ≤ x ≤ 1) semiconductor nanocrystals

Semiconducting CuIn_xGa_1−xSe₂ nanocrystals (20–30 nm) have been synthesized over the whole composition range using a facile solution-based method. Depending on the synthesis conditions, the nanocrystals exhibit a cubic or spherical morphology with a narrow size distribution. The band gap increases with increasing Ga concentration and the values are close to those observed in the bulk.     

Synthesis and low temperature fluorination of the alkaline-earth palladates Ba2−xSrxPdO3 (x = 0–2)

Since the discovery of superconductivity in Sr₂CuO₂F_2+δ there has been an increased interest in ternary oxide-fluorides. Sr₂CuO₂F_2+δ is prepared via low temperature (T = 220 °C) reaction routes. Low temperature fluorination induces an interesting structural rearrangement in the parent compound Sr₂CuO₃, which is a one-dimensional material containing linear chains of vertex sharing CuO₄ squares along the crystallographic b axis. Upon fluorination, one oxide is substituted by two fluorides and Cu²⁺ becomes octahedrally coordinated by four oxides and two fluorides. The fluorinated compound Sr₂CuO₂F_2+δ displays the T-type structure (La₂CuO₄). Insertion of excess fluorine, δ, also takes place and this fluorine occupies interstitial sites in the T structure. Although the starting material Ca₂CuO₃ is isostructural to Sr₂CuO₃, Ca₂CuO₂F_2+δ displays the T′ (Nd₂CuO₄) structure due to the smaller radius of Ca²⁺ compared to that of Sr²⁺.

The alkaline-earth palladates with the general formula A₂PdO₃ (A = Ba, Sr) are isostructural with the A₂CuO₃(A = Ca, Sr) materials. We prepared the Ba_2−xSr_xPdO₃ (x = 0–2) series and performed low temperature fluorination, which led to the synthesis of the series Ba_2−xSr_xPdO₂F_2+δ (0 ≤ x ≤ 1.5). All the compounds in the Ba_2−xSr_xPdO₂F_2+δ series show T′ structure (Ca₂CuO₂F_2+δ). Similarities and differences with Sr₂CuO₂F_2+δ and Ca₂CuO₂F_2+δ will be discussed.     

La1-xSrxCoO3（x=0，0.2，0.4，0.6，0.8）钙钛矿型氧化物的催化性能及改性

通过溶胶-凝胶法制备出A位Sr掺杂的钙钛矿型氧化物La_1-xSr_xCoO₃（x=0,0.2,0.4,0.6,0.8）,并将其作为催化剂应用于双功能氧电极中。测试结果表明,A位Sr的掺杂的La_1-xSr_xCoO₃比LaCoO₃具有更高的电催化活性,并且La_0.6Sr_0.4CoO₃在氧还原和氧析出反应中均表现出最优的催化性能,最大电流密度分别达到0.244 A·cm^-2（-0.6 V vs Hg/HgO）和0.303 A·cm^-2（1 V vs Hg/HgO）。为进一步提高催化剂的催化活性,将水热法制备的α-MnO₂纳米管与La_0.6Sr_0.4CoO₃复合作为双功能催化剂。当α-MnO₂的质量分数为40%时,比起单一的α-MnO₂或钙钛矿氧化物,α-MnO₂/La_0.6Sr_0.4CoO₃复合材料表现出协同效应,有更好的双功能电催化活性,使双效氧电极具有更好的电化学性能及稳定性。     

La1-xSrxCoO3(x=0,0.2,0.4,0.6,0.8)钙钛矿型氧化物的催化性能及改性

通过溶胶-凝胶法制备出A位Sr掺杂的钙钛矿型氧化物La_(1-x)Sr_xCoO_3(x=0,0.2,0.4,0.6,0.8),并将其作为催化剂应用于双功能氧电极中。测试结果表明,A位Sr的掺杂的La_(1-x)Sr_xCoO_3比LaCoO_3具有更高的电催化活性,并且La_(0.6)Sr_(0.4)CoO_3在氧还原和氧析出反应中均表现出最优的催化性能,最大电流密度分别达到0.244 A·cm~(-2)(-0.6 V vs Hg/HgO)和0.303 A·cm~(-2)(1 V vs Hg/HgO)。为进一步提高催化剂的催化活性,将水热法制备的α-MnO_2纳米管与La_(0.6)Sr_(0.4)CoO_3复合作为双功能催化剂。当α-MnO_2的质量分数为40%时,比起单一的α-MnO_2或钙钛矿氧化物,α-MnO_2/La_(0.6)Sr_(0.4)CoO_3复合材料表现出协同效应,有更好的双功能电催化活性,使双效氧电极具有更好的电化学性能及稳定性。     

Preparation and electrical properties of new oxide ion conductors Ce6−xDyxMoO15−δ (0.0 ≤ x ≤ 1.8)

Ce_6−xDy_xMoO_15−δ (0.0 ≤ x ≤ 1.8) were synthesized by modified sol–gel method. Structural and electrical properties were investigated by means of X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS). The XRD patterns showed that the materials were single phase with a cubic fluorite structure. Impedance spectroscopy measurement in the temperature range between 350 °C and 800 °C indicated a sharp increase in conductivity for the system containing small amount of Dy₂O₃. The Ce_5.6Dy_0.4MoO_15−δ detected to be the best conducting phase with the highest conductivity (σ_t = 8.93 × 10⁻³ S cm⁻¹) is higher than that of Ce_5.6Sm_0.4MoO_15−δ (σ_t = 2.93 × 10⁻³ S cm⁻¹) at 800 °C, and the corresponding activation energy of Ce_5.6Dy_0.4MoO_15−δ (0.994 eV) is lower than that of Ce_5.6Sm_0.4MoO_15−δ (1.002 eV).     

Synthesis, structure and oxygen nonstoichiometry of La0.4Sr0.6Co1−yFeyO3−δ

The samples of La_0.4Sr_0.6Co_1−yFe_yO_3−δ (y = 0.2 and 0.4) were prepared using both conventional ceramic technique and nitrate–citrate precursors technique. The phase identification was made by X-ray diffraction method. The refinement of structural parameters from the XRD and neutron diffraction measurements was performed by full profile Rietveld analysis. Neutron diffraction showed that both samples possess distorted perovskite-type structure. Oxygen nonstoichiometry was measured by chemical analysis and thermogravimetry (TG) analysis in the range 20 ≤ T/°C ≤ 900 and 2E-5 ≤ pO₂/atm ≤ 4E-1. TG-experiments indicate a relatively fast and reversible oxygen exchange at pO₂ > 1E-2 atm. Mass saturation occurs at T < 300 °C upon cooling. The absolute value of oxygen nonstoichiometry was determined by iodometric titration measurements. It was found that both samples have practically stoichiometric composition at 300 °C in air and δ increases with increasing temperature and decreasing oxygen partial pressure.     

(La0.4Ba0.4Ca0.2)(Mn0.4Ti0.6)O3: A new titano-manganate with a high dielectric constant and antiferromagnetic interactions

A new perovskite-based titano-manganate, (La_0.4Ba_0.4Ca_0.2)(Mn_0.4Ti_0.6)O₃, has been prepared by the ceramic route at 1100°C. This oxide was found to possess the cubic perovskite structure with  Å (space group ). The refined composition as obtained by Rietveld analysis of powder X-ray data was found to be (La_0.44Ba_0.38Ca_0.18)(Mn_0.43Ti_0.57)O_2.91(3) (R_p=0.0704, wR_p=0.0828). The composition was also ascertained by Energy dispersive X-ray analysis. Iodometric studies led to a slightly higher oxygen content (compared to Rietveld refinement) corresponding to an average manganese oxidation state of 3.05. The above oxide was found to exhibit high dielectric constant (ε) of 6980 at 1 kHz decreasing to 590 at 100 kHz. At high temperatures (200°C) it shows an unusually high dielectric constant of 20,000 at 1 kHz. In addition to the dielectric properties, detailed magnetic studies show evidence of long-range antiferromagnetic interactions near 5 K. The presence of unusually high dielectric constant coupled with the long-range magnetic interactions may open up interesting applications.     

Oxygen permeation through a Ce0.8Sm0.2O2−δ–La0.8Sr0.2CrO3−δ dual-phase composite membrane

A composite of oxygen ion conducting oxide Ce_0.8Sm_0.2O_2−δ (60 vol.%) and electron conducting oxide La_0.8Sr_0.2CrO_3−δ was prepared by sintering a powder compact at a temperature of 1550 °C. No significant reaction between the two constituent oxides was observed under preparation and oxygen permeation conditions. Appreciable oxygen permeation fluxes through the composite membrane were measured at elevated temperatures with one side of it exposed to the ambient air and the other side to a flowing helium gas stream. The oxygen flux initially increased with time, and took a long time to reach a steady value. A steady oxygen permeation flux as high as 1.4 × 10⁻⁷ mol cm⁻² s⁻¹ was obtained with a 0.3 mm thick membrane at 950 °C under a relatively small oxygen partial pressure difference of 0.21 bar/0.0092 bar. It was revealed that the overall oxygen permeation process was mainly limited by the transport in the bulk of the membrane in the range of the membrane thickness greater than 1.0 mm, and the limitation by the surface oxygen exchange came into play at reduced thickness of 0.6 mm.     

A TEM, XRD, and crystal chemical investigation of oxygen/vacancy ordering in (Ba1−xLax)2In2O5+x, 0x0.6

A careful TEM and XRD study of the (Ba_1−xLa_x)₂In₂O_5+x, 0x0.6, ‘defect-perovskite’-type solid solution has been carried out. A well-defined structural phase transition is shown to occur between x=0.1 and 0.2 from the orthorhombic brownmillerite structure type on the low x side to a multiple twinned, tetragonal 1×1×2 perovskite-related superstructure phase on the high x side at x=0.2. This phase transition correlates with an important phase transition previously observed in electrical conductivity versus temperature measurements. The existence of additional satellite reflections close to the regions of reciprocal space was found to be typical of all (Ba_1−xLa_x)₂In₂O_5+x specimens, although their intensity relative to the parent Bragg reflections systematically reduces as x increases. As x increases beyond 0.2, the -type satellite reflections initially become weaker and rather more diffuse for x=0.3 before splitting into pairs of rather weak and somewhat diffuse incommensurate satellite reflections for x=0.4 and beyond. An interpretation in terms of oxygen vacancy ordering and associated structural relaxation is given. Additional structured diffuse scattering is also observed and a tentative explanation in terms of Ba/La ordering and associated local strain distortions put forward.     

Crystal structure and electrical conductivity of lanthanum-calcium chromites-titanates La1−xCaxCr1−yTiyO3−δ (x=0-1, y=0-1)

Five series of perovskite-type compounds in the system La_1−xCa_xCr_1−yTi_yO₃ with the nominal compositions y=0, x=0-0.5; y=0.2, x=0.2-0.8; y=0.5, x=0.5-1.0; y=0.8, x=0.6-1.0 and y=1, x=0.8-1 were synthesized by a ceramic technique in air (final heating 1350 °C). On the basis of the X-ray analysis of the samples with (Ca/Ti)?1, the phase diagram of the CaTiO₃-LaCr^IIIO₃-CaCr^IVO₃ quasi-ternary system was constructed. Extended solid solution with a wide homogeneity range is formed in the quasi-ternary system CaCr^IVO₃-CaTiO₃-LaCr^IIIO₃. The solid solution La_{(1−x′−y)}Ca_(x′+y)Cr^IV_x′Cr^III_{(1−x′−y)}Ti_yO₃ exists by up to 0.6-0.7 mol fractions of CaCr^IVO₃ (x^′<0.6-0.7) at the experimental conditions. The crystal structure of the compounds is orthorhombic in the space group Pbnm at room temperature. The lattice parameters and the average interatomic distances of the samples within the solid solution ranges decrease uniformly with increasing Ca content. Outside the quasi-ternary system, the nominal compositions La_0.1Ca_0.9TiO₃, La_0.2Ca_0.8TiO₃, La_0.4Ca_0.6Cr_0.2Ti_0.8O₃ and La_0.3Ca_0.7Cr_0.2Ti_0.8O₃ in the system La_1−xCa_xCr_1−yTi_yO₃ were found as single phases with an orthorhombic structure. In the temperature range between 850 and 1000 °C, the synthesized single-phase compositions are stable at pO₂=6×10⁻¹⁶-0.21×10⁵ Pa. Oxygen stoichiometry and electrical conductivity of the separate compounds were investigated as functions of temperature and oxygen partial pressure. The chemical stability of these oxides with respect to oxygen release during thermal dissociation decreases with increasing Ca-content. At 900 °C and oxygen partial pressure 1×10⁻¹⁵-0.21×10⁵ Pa, the compounds with x>y (acceptor doped) are p-type semiconductors and those with x<y (donor doped) and x=y are n-type semiconductors. The type and level of electrical conductivity are functions of the concentration ratios of cations occupying the B-sites of the perovskite structures: [Cr³⁺]/[Cr⁴⁺] and [Ti⁴⁺]/[Ti³⁺]. The maximum electrical conductivity at 900 °C and pO₂=10⁻¹⁵ Pa was found for the composition La_0.1Ca_0.9TiO₃ (near 50 S/cm) and in air at 900 °C for La_0.5Ca_0.5CrO₃ (close to 100 S/cm).     

Pressure Dependence of the Lattice Distortion in Perovskite La0.5−xBixCa0.5MnO3 (x=0.1, 0.15, 0.2)

The in situ behavior of distorted perovskite La_0.5−xBi_xCa_0.5MnO₃ (x=0.1, 0.15, 0.2) under high pressure has been studied by energy-dispersive X-ray diffraction in a diamond anvil cell. An abnormal change of the 202–040 d-spacing ascribed to the disappearance of the distortion mode Q₂ in the MnO₆ octahedra is observed at 1.2, 1.4, and 1.6 GPa, respectively, and it results in a reduction of the Jahn–Teller distortion commonly existing in the manganites. Effect of the unique 6s² long-pair character of the Bi³⁺ ion on the pressure dependence of the lattice distortion is discussed.     

层状钙钛矿型Sm2-2xSr1+2x-2yCa2yMn2O7(x=0.2，0.3，0.4，0.5；y=0，0.2，0.3)的溶胶凝胶合成及电性能研究

Layered perovskite manganites with a nominal chemsitry of Sm_2-2xSr_1+2x-2_yCa_2yMn₂O₇(x=0.2, 0.3, 0.4, 0.5; y=0, 0.2, 0.3) were prepared using sol-gel method. The crystal structures of these compounds were studied by X-ray diffraction (XRD) and FTIR absorption spectra. The absorption peaks become weaker and move a little to higher frequency with increasing of Sm/Sr concentration. As the Sm doping increases to x=0.2 and x=0.3, the absorption peaks show a cubic structure character, reflecting that the samples suffer from a transition from tetragonal structure to cubic structure. This coincides with the X-ray diffraction results. The high temperature electrical properties were studied by conventional four-probe method. Although all samples exhibit the semiconductive behavior, lnρ-1 000/T curves are not linear and they obey the small polaron hopping mechanism. Moreover, the resistivity decreases with x reducing. This is due to that Sm doping increases the Jahn-Teller ion Mn³⁺/Mn⁴⁺ ratio, and decreases the e_g bandwidth.     

IR-photodissociation and photodetachment spectroscopy of Cl · (NH3)x (IR: x = 1–4, PD: x = 1)

Complexes of ammonia molecules and one chloride ion have been studied by photodetachment and IR-photodissociation spectroscopy. For the smallest anionic complex, the stabilisation energy with respect to the bare chloride ion and vibrational frequencies have been determined. Two bands showed a splitting due to rotational branches, which could be represented by simulation. Rotational constants obtained by former ab initio calculations [P.S. Weiser, D.A. Wild, P.P. Wolynec, E.J. Bieske, J. Phys. Chem. A 104 (2000) 2562] are confirmed and rotational constants of a vibrationally excited state are supplied. IR-photodissociation spectra of clusters with up to four ammonia molecules per chloride ion were recorded.     

Properties of oxygen permeation and partial oxidation of methane in La0.6Sr0.4CoO3−δ (LSC)–La0.7Sr0.3Ga0.6Fe0.4O3−δ (LSGF) membrane

The oxygen separation membrane having perovskite structure for the partial oxidation of methane to synthesis gas was prepared. La_0.7Sr_0.3Ga_0.6Fe_0.4O_3−δ (LSGF) perovskite membrane coated with La_0.6Sr_0.4CoO_3−δ (LSC) (M1), and the one side of M1 membrane coated with NiO (M2) was prepared to examine the partial oxidation of methane. The single oxygen permeations of the LSC + LSGF (M1) membrane and NiO coated membrane (M2) were measured. The oxygen permeation flux in M1 membrane was higher than that of M1 membrane at 850 °C.

The partial oxidation experiment of methane using the prepared membranes was examined at 850 °C. The value of CH₄ conversion and CO selectivity of M2 membrane was higher than that of M1 membrane.

NiO/NiAl₂O₄ catalyst was used to improve the methane conversion, and the partial oxidation experiment of methane with M1 membrane was examined at 850 °C. The CH₄ conversion was 88%, and CO selectivity was 100%.     

Magnetic transitions in the Gd5Si4−xPx (x = 0.5, 0.75, 1.25) phases. Magnetocaloric effect of the Gd5Si2.75P1.25 phase

The Gd₅Si_2.75P_1.25 phase with all interslab Si/P–Si/P dimers broken (Sm₅Ge₄-type structure) undergoes a ferromagnetic transition at 184 K. For this phase, the magnetocaloric effect in terms of the magnetic entropy change, ΔS, reaches the maximum value of −7.8 J/kg K at 177 K. Absence of a temperature-dependant structural transition, as confirmed by the low-temperature single crystal diffraction studies, together with the moderate value of ΔS points to the presence of a conventional magnetocaloric effect. Gd₅Si_3.5P_0.5 and Gd₅Si_3.25P_0.75, which are composites of the Gd₅Si₄- (all Si/P–Si/P dimers intact) and Sm₅Ge₄-type phases, possess two magnetic transitions associated with the two-phases. Introduction of P into Gd₅Si₄ lowers the Curie temperature from 336 K to 332 K in Gd₅Si_3.25P_0.75.